1. Field of the Invention
The present invention relates to a method and an apparatus for angularly indexing a crankshaft to plural angular positions successively in order to selectively position plural pin portions of the crankshaft in a predetermined machining position aligned with the rotational axis of a work spindle.
2. Discussion of the Prior Art
In a general pin grinder, it is required to index a crankshaft on its chucking device in order to selectively locate plural pin portions thereof at a predetermined machining position, at which the selected pin portion is aligned with the rotational axis of the work spindle, before a clamping operation of the crankshaft with respect to the chucking device. Since the positioning error during the index operation affects the grinding accuracy of the pin portions, the indexing of the crankshaft must be accomplished accurately. Therefore, prior pin grinders are provided with various kinds of indexing mechanisms capable of indexing a crankshaft accurately.
For example, a pin grinder disclosed in the Japanese unexamined patent application 60-118403 is provided with a reference member fixed to its chucking device for engagement with a balance weight portion of a crankshaft, and a swing arm pivoted on its bed for swing movement. In the pin grinder, the swing arm is engaged with one of pin portions in order to rotate the crankshaft about its journal axis until the balance weight portion of the crankshaft engages with the reference member. With this operation, the pin portion is accurately positioned at the machining position aligned with the work spindle.
On the other hand, in a pin grinder disclosed in the Japanese unexamined patent application 57-21261, a shoe of the rest device is used as a reference member, and a pin portion to be positioned in the machining position is engaged with the shoe by a rotational movement of a crankshaft, so that the selected pin portion is located at the machining position.
Although those two pin grinders can index a crankshaft accurately, they have a disadvantage that the total machining cycle time becomes long, because the time consuming indexing operation must be repeated before each grinding operation for respective pin portions.
To obviate above-mentioned problems, a mechanism disclosed in the Japanese unexamined patent application 62-15070 can be used. The pin grinder is provided with a mechanism for indexing a crankshaft with a phase indexing motor and a fine positioning mechanism. In the pin grinder, each of the pin portions are roughly indexed to the machining position by actuating indexing motor, then precisely positioned by actuating the positioning mechanism. Namely, the chucking device is provided with an indexing plate engaging with a hole formed at one end of the crankshaft through an engaging pin, and a positioning piston whose piston rod is engageable with engagement holes formed on the periphery of the indexing plate when the piston is advanced. The indexing plate is first rotated by a predetermined angle by the indexing motor so that one of the pin portions is roughly positioned in the machining position. After that, the positioning piston is advanced so that the piston rod engages with one of engagement holes, resulting in accurate positioning of the crankshaft. After such indexing operation, the crankshaft is clamped with respect to the chucking device. As is understood from the above-explanation, since the pin grinder must be provided with a positioning mechanism, i.e., indexing plate with plural engagement holes and positioning piston, for fine positioning, it has a disadvantage that structure of the chuck device becomes complex. Further, if there is a clearance between the engaging pin and the engagement hole of the crankshaft, pin portions cannot be indexed accurately to the machining position even if the chucking device is provided with the above-mentioned fine indexing mechanism.